1. Field of the Invention
The present invention relates to an integrated circuit (thin film integrated circuit) formed by using a semiconductor film which has a crystal structure and is formed on a glass substrate, and more particularly, the invention relates to a thin film integrated circuit in which a signal is transmitted by means of an optical interconnect, and to a semiconductor device using the relevant thin film integrated circuit.
2. Description of the Related Art
As the minimum dimension for LSI approaches a sub-micron region, a high integration of LSI has been limited. In the case where a sufficient high integration is not realized in an integrated circuit, the wiring resistance is heightened. Therefore, the delay of signals and the disturbance of waveforms occur, the amount of transmission of signals is decreased, and the performance of the information processing of the integrated circuit becomes restricted. Moreover, as wirings are lengthened, a parasitic capacitance between the wirings is increased, and charging and discharging of energy to the wirings are also increased, thus electric power consumption cannot be lowered.
Therefore, a technology for forming a three-dimensional integrated circuit attracts the attention in order to realize higher integration. The three-dimensional integrated circuit is formed by stacking semiconductor elements with an interlayer insulating film interposed therebetween, and this technology is employed to realize much higher integration, higher data rates due to the shortening of the wirings, and multi-function operation of the integrated circuit.
In order to form the three-dimensional integrated circuit, each semiconductor element formed on each layer has to be electrically interconnected. The electrical interconnection between semiconductor elements can be performed by wire bonding or flip chip bonding, or by forming a via hole in an interlayer insulating film which separates each layer. Further, what is called an optical interconnect can be employed, in which an electric signal is converted to an optical signal by using a light emitting element and the semiconductor elements are interconnected through optical transmission.
A technology for data transmission between a CPU, a memory controller and a memory by using an optical signal is disclosed in Patent Document 1 below.
[Patent Document 1]
Japanese Patent Laid-Open No. 2000-58882
As described in Patent Document 1, unnecessary radio wave radiation and skew between data caused by the wiring resistance can be suppressed by means of data transmission using an optical signal.
As mentioned above, three-dimensionalization of the integrated circuit is an essential technology for realizing higher integration. However, as the number of layers is increased so as to increase the amount of information to be processed by a semiconductor device, it becomes more difficult to interconnect each layer.
In the case of using the wire bonding, for example, the adjacent layers are easily interconnected with a thin metal wire, but in the case where two layers to be interconnected are separated from each other, the form of layers interposed between the two layers is restricted in order not to obstruct the formation of a thin metal wire used for interconnecting. Therefore, when semiconductor elements are multilayered and the two layers are interconnected with more layers interposed therebetween, connection with a thin metal wire becomes difficult. Further, the bonding with a thin metal wire is commonly performed in the vicinity of the edge of each layer, thus the area for connecting the thin metal wire is restricted and the number of terminals of each layer cannot be easily increased.
Meanwhile in the case of using the flip chip bonding, since the area for electrically connecting is not restricted, the number of terminals of each layer can be relatively increased as compared with in the case of the wire bonding. As well as the wire bonding, the flip chip bonding is suitable for interconnecting the adjacent layers. However, when two layers to be interconnected are separated from each other, the form of layers interposed between the two layers is restricted in order not to obstruct the connection with a solder ball, therefore, the flip chip bonding is unsuitable for multilayering as well as the wire bonding.
When interconnecting two layers by forming a via hole in an interlayer insulating film therebetween, and the two layers to be interconnected are separated from each other, the via hole has to be formed so as to penetrate all the layers placed between the two layers. The thicker the layers to be penetrated are, the longer the via hole diameter is. Accordingly, high integration of wirings is prevented and this is also unsuitable for mulilayering.
In the case of using the optical transmission, unlike the above-mentioned cases, two layers can be interconnected without restricting the form of the other interposed layers therebetween by using an optical system such as an optical fiber, even when the two layers to be interconnected are separated from each other. However, since the optical fiber is very expensive in general, it is not suitable for a semiconductor device using an integrated circuit in view of a cost thereof.
When a plurality of thin film integrated circuits are sequentially formed to be layered on a substrate, optical transmission can be easily performed without an optical system such as an optical fiber. However, crystallization of a semiconductor film used for a semiconductor element of the upper layer has to be carried out at a temperature equal to or lower than a heat-resistance temperature of the lower layer. Thus, there are many restrictions on manufacturing steps, and it is difficult to form a thin film integrated circuit with a higher performance.
In view of the foregoing, it is the primary object of the invention to provide a semiconductor device, which has a high performance integrated circuit formed on an inexpensive glass substrate and capable of processing the large amount of information, operating at higher data rates.